In recent years, such a method (a plutonium-thermal method) has been proposed that fuel assemblies for a PWR (Pressurized Water Reactor) or a BWR (Boiling Water Reactor) having used for nuclear power generation are recovered and stored in a radioactive substance storage container, transported to storage facilities, kept therein for a certain period of time, and then recycled. In such a radioactive substance storage container, it is desired to further improve the subcritical safety thereof in storing fuel such as high-burnup spent fuel, fuel having uranium enriched to a degree higher than the existing fuel, or MOX fuel (Mixed Oxide Fuel) in the radioactive substance storage container.
To resolve this issue, a conventional radioactive substance storage container includes a body and a lid having a sealed structure, and a basket stored in the body to hold the fuel assemblies. The basket has neutron shielding performance, thereby increasing subcritical safety of the container. A technique described in Patent Literature 1 is known as an example of the conventional radioactive substance storage container with this configuration.